In general, micro circuits such as integrated circuits are fabricated from thin semiconductor slices--called wafers--having a plurality of micro circuits or chips, thereon. Customarily, each wafer contains a plurality of identical repeating matrices of the same type of micro circuit. Before final processing and encapsulation, it is customary to test each chip prior to separating by dicing the wafer into desired individual chips. Since each chip is normally positioned in a predetermined precise relation with respect to adjacent circuit units, it is possible to automatically test each individual chip separately. A plurality of probes have to be accurately located on each preselected contact pad of the chip to be tested on the wafer. Normally, testing is performed by applying signals at input terminals of the chip and measuring the values at the output terminals of the chip via the probes.
Several obstacles must be overcome in this test procedure in order to have reliable testing. Since the relative height of the contact pads of a chip on a wafer can vary substantially from one pad to the next, testing probes having electrodes with mechanically flexible characteristics have been employed in the prior art in order to properly contact the pads. Wherever the chip pads have a substantial dimensional tolerance in the relative height thereof with respect to the surface of the wafer, the resiliency of the probes being incorporated in a test head is to compensate for such variation. However, a problem arises when the resiliency of the probes may not be sufficient to compensate for the dimensional tolerance of the pads, since the probes themselves also have a tolerance in length relatively to the test head. In the case that a relatively short probe is to contact a relatively flat pad, an open may exist between the probe and the pad. Since it is not possible to distinguish a real open in the chip to be tested and a pseudo open between a probe and a pad, the result of the test procedure may be erroneous thereby resulting in an actually good chip being considered not operational. Obviously this results in a loss of yield. Test systems of this kind have been proposed, for example, in IBM Technical Disclosure Bulletin, Volume 22, No. 7, December 1979, pages 2824 to 2826, in IBM Technical Disclosure Bulletin, Volume 26, No. 1, June 1983, page 251, and in European Patents EP-B-0 130 350, EP-A-0 331 163, EP-A-0 256 541 and EP-A-0 343 021, wherein different resilient probe constructions are shown.
In order to improve the reliability of testing, flexible probes having a piezoelectric member have been described in U.S. Pat. No. 4,951,370. An electrical signal is produced by the piezoelectric member when sufficient force is imparted to the probe tip to cause corresponding flexing of the piezoelectric member. This signal serves to indicate whether mechanical contact of probe and pad has been achieved. This control signal is also used according to the teaching of this document for a technique called scrub-in. After the probe tips make contact with the pads, it is necessary for the wafer-holder to move an additional distance towards the probes in order to break through the oxide layer on the pads and make good electrical contact with the chip. However special piezoelectric probes have to be used according to the teaching of this document and no solution to the above described problem is provided, when a relatively short probe is to contact a relatively flat pad so that an open in the chip is indicated where in reality none exists.